State-Specific Changes in Singleton Preterm Births Among Black and White Women --- United States, 1990 and 1997

National infant mortality rates among non-Hispanic black women are twice those of non-Hispanic white women (1). Nearly two-thirds of this disparity is attributable to a higher rate of preterm delivery (PTD) (i.e., <37 weeks' gestation) among blacks (2). To investigate state-specific changes in PTD rates among blacks and whites, natality data for 1990 and 1997 were analyzed from 50 states and the District of Columbia (DC). These data indicated that, although the PTD rate was twice as high among blacks than among whites, the disparity decreased as the result of an increase in preterm births among whites and a decrease among blacks (3).

U.S. natality files for 1990 and 1997 were used for this analysis. PTD was defined as a singleton, live birth occurring at 17--36 weeks' gestation. Gestational age was determined using the first day of the mother's last normal menstrual period (LMP) and the date of delivery. A clinical estimate of gestational age was used when the month or year of LMP was missing or gestational age based on LMP was inconsistent with the infant's birth weight (4). Approximately 1% of singleton infants were excluded because of missing data. Maternal race/ethnicity was based on self-report recorded on the infant birth certificate. PTD rates were determined for each state and DC for 1990 and 1997. Rates were not calculated for reporting areas with <20 PTDs. Standard errors were calculated for each rate, and Z scores were used to assess statistically significant rate changes (5).

Overall, an 11% increase in PTDs occurred among whites; significant changes were reported in 38 states. DC alone showed a PTD decline among whites. In 1990, the PTD rate among whites was 75.4 per 1000 live births (range: 56.6--103.0 live births), and 178.5 (range: 113.5--228.2 live births) among blacks. In 1997, the PTD rate among whites increased to 83.7 (range: 65.4--106.7). Among blacks, the 1997 national PTD rate decreased 10% to 160.9 (range: 108.8--197.3). From 1990 to 1997, 24 states showed significant declines in PTD rates among blacks. In 1997, West Virginia had the highest preterm birth rate among whites (106.7) and Minnesota had the lowest PTD rate among blacks (108.8) (Table 1).

In 1990, 35 (81%) of 41 states and DC had a black-to-white PTD rate ratio (RR) of >2.0; seven (19%) had a RR of 1.6--1.9 (Table 1). In 1997, reporting areas with a RR >2.0 decreased to 11 (26%) of 43 (Oklahoma was an added reporting state). Thirty-two (74%) of 43 reporting areas had a RR of 1.4--1.9. No reporting area had a RR of 1.0 (i.e., indicating no disparity between groups). Changes in the RR for individual states occurred because of decreases among blacks and increases among whites in 21 states (Colorado, Delaware, Florida, Illinois, Louisiana, Maryland, Michigan, Minnesota, Nebraska, Nevada, New York, North Carolina, Ohio, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, Washington, and Wisconsin); decreases among blacks and unchanged rates among whites occurred in two states (California and Georgia); unchanged rates among blacks and increases among whites occurred in 13 states (Alabama, Arizona, Connecticut, Indiana, Iowa, Kansas, Kentucky, Massachusetts, Mississippi, Missouri, New Jersey, Utah, and West Virginia); and decreases occurred among blacks and whites in DC.

Reported by: D Taylor, California Dept of Health Svcs. Pregnancy and Infant Health Br, Div of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion; State Br, Div of Applied Public Health Training, Epidemiology Program Office; Reproductive Statistics Br, Div of Vital Statistics, National Center for Health Statistics; and an EIS Officer, CDC.

Editorial Note:

The PTD disparity in the United States has narrowed between blacks and whites nationally and in several states; however, a 1.5--2.4-fold excess risk for PTD among blacks remains a public health concern if the 2010 national goal of eliminating PTD disparities among U.S. racial/ethnic groups is to be reached.

Although the etiology of PTD is unclear, some known risk factors include maternal conditions, infection, stress, smoking, previous PTD, maternal age, and other demographic factors. The higher risk for PTD among blacks may reflect a greater prevalence and/or severity of these risk factors, and less access to health care and resources. Although this report did not examine the reasons for these decreases in black PTD and increases in white PTD, previous analyses showed that changes in the maternal age distribution, time of entry into prenatal care, marital status, medical induction rates, and method of estimation of gestational age explained some, but not all, of the observed trends (6). State-specific analyses using data from sources such as the Pregnancy Risk Assessment Monitoring System may provide insight into additional factors that contribute to the reported trends.

The findings in this study are subject to at least three limitations. First, errors in LMP or clinically estimated gestational age may have resulted in misclassification of preterm status (e.g., imperfect maternal recall, postconception bleeding, delayed ovulation, or intervening early miscarriage). Such errors may occur more frequently in some populations, especially when gestation has been brief (7). Second, changes in the reporting of preterm live birth with the shortest gestations could have affected the PTD rates (8). However, such births represented a small fraction of total PTD and may not have contributed to overall trends. Third, because fetal deaths were not evaluated, the contribution of changes in fetal survival to the increase in PTD could not be assessed.

Research is needed into the biologic, psychological, social, economic, and environmental factors that contribute to PTD. Progress in reducing PTD in all states will require more support for implementing the three components of Safe Motherhood (i.e., prevention research, population-based health monitoring, and effective prevention programs) (9). Although prenatal care can address modifiable risk factors, reducing PTD and eliminating racial/ethnic disparities may entail interventions at multiple levels, including individual patients and health-care providers, systems of care, and social policies.

References

1. MacDorman MF, Atkinson JO. Infant mortality statistics from the period 1997 linked birth/infant death data set. Hyattsville, Maryland: US Department of Health and Human Services, CDC, National Center for Health Statistics, 1999 (National Vital Statistics Reports, vol 47, no. 23).
2. Iyasu S, Becerra JE, Rowley DL, Hogue CJ. Impact of very low birthweight on the black-white infant mortality gap. Am J Prev Med 1992;8:271--7.
3. CDC. Preterm singleton births---United States, 1989--1996. MMWR 1999;48:185--9.
4. Ventura SJ, Martin JA, Curtin SC, Mathews TJ. Report of final natality statistics, 1996. Mon Vital Stat Rep 1998;46(suppl 11).
5. Hoyert DL, Kochanek KD, Murphy SL. Deaths: final data for 1997. Hyattsville, Maryland: US Department of Health and Human Services, CDC, National Center for Health Statistics, 1999 (National Vital Statistics Reports, vol 47, no. 19).
6. CDC. Preterm singleton births---United States, 1989--1996. MMWR 1999;48:185--9.
7. Alexander GR, Himes JH, Kaufman RB, Mor J, Kogan MA. United States national reference for fetal growth. Obstet Gynecol 1996;87:163--8.
8. Phelan ST, Goldenberg R, Alexander G, Cliver SP. Perinatal mortality and its relationship to the reporting of low-birthweight infants. Am J Public Health 1998;88:1236--9.
9. CDC. World health day---April 7, 1998. MMWR 1998;47:218.

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